Field of the Invention
The present invention relates generally to image formation devices, and in particular to a coupling mechanism between an imaging unit and its drive source in a color electrophotographic printer; specifically, an Oldham coupler for transferring rotary power between two parallel non-collinearly aligned shafts.
Description of the Related Art
Oldham couplers have been employed for decades in drive systems of diverse machines for transferring torque or rotary power between two parallel but non-collinear and/or non-radially aligned rotating shafts. In the past, Oldham couplers have been used to couple two shafts together. Further retraction splines have been provided on Oldham couplers, and such retraction splines travelled through the center of the gearing which limited their size and torsional stiffness.
Monochromatic or color electrophotographic printers appear to be sensitive to low frequency oscillations of the drive system. As the Oldham coupler oscillates when driving the imaging units of the printer, banding may occur on the printed media sheet with band heights in the range of 0.5 mm to 2 mm. By using a stiffer drive system, the natural frequency of the drive system will increase and shorten the wavelength of the banding to the point that it is not visible on the page to the naked eye. In the past Oldham couplers with retraction splines made of all plastic experienced this problem. A combination of metal input and output plates with a plastic star plate has been used to improve the torsional stiffness of the Oldham coupler while reducing the amount of banding on the printer page. However, such combination Oldham couplers are relatively expensive.
As process speeds increase in printers, motors speeds are limited. This forces gear ratios to be lowered with increased spacing of the gear teeth with respect to the media sheet being printed. This increase gear teeth spacing will make fine line jitter more apparent to the naked eye. Slower printers have used higher gear ratios to the motor with gear teeth spacing as low as 0.2 mm. These machines were less likely to have jitter issues. Faster printers are more sensitive to the stiffness of the drive system and visual jitter.
Also, with prior Oldham coupler designs, the input and output plates of the coupler engaged with the two shafts on which the gears were mounted. This arrangement increased the overall axial width of the combination. Further the room needed for the stud or screw attachment of the wheels used in the prior art Oldham coupler adds to the width. This leads to a smaller wheel to axle ratio and a higher side load generated by the Oldham coupler.
It would be advantageous to have an Oldham coupler that provides increased torsional stiffness but at a lower cost than current combination Oldham couplers. It would be further advantageous to have an Oldham coupler having a reduced side load. It would be further advantageous to be able to have a plastic retraction spline having increased torsional stiffness.